1. Field of the Invention
This invention relates to a process for the preparation of O-acetyl compounds such as acetic anhydride by the catalytic reaction of O-methyl compounds, such as methyl acetate, as starting material, with carbon monoxide, using rhodium as a main catalyst.
Acetic anhydride is used in a large amount as a material for the production of cellulose acetate and further is useful as a material for pharmaceuticals, perfumes, dyestuffs and so on.
2. Description of Prior Art
Acetic anhydride has been conventionally produced on an industrial scale by the reaction of acetic acid with ketene obtained by thermal cracking of acetic acid.
On the other hand, in research of so-called C.sub.1 chemistry, the production of acetic anhydride by the reaction of carbon monoxide with methyl acetate or dimethyl ether has been actively attempted. Particularly, in a method in which rhodium is used as a main catalyst (Japanese Patent Laid-open No. 30820/1975), the reaction proceeds under milder conditions than those in which other transition metal catalysts are used, but this method is still insufficient industrially in the reaction rate, and hence improved methods in which various reaction accelerators are added to the rhodium catalyst system have been proposed.
It is known that, in the carbonylation of methyl acetate to acetic anhydride, numerous kinds of metals act as an accelerator effective for enhancing the catalytic activity of the rhodium catalysts when added to rhodium-iodine compounds, typically, methyl iodide catalyst systems. According to Japanese Patent Laid-open No. 52017/1975 (corresponding to British Pat. No. 1468940), elements having atomic weights of at least 5 among those of Groups IA, IIA, IIIA, IVB, and VIB, non-noble metals of Group VIII, lanthanide elements, and actinide elements are considered to be effective, and particularly lithium is given as a preferred metal. Japanese Patent Laid-open No. 47922/1975 discloses a method wherein 14 kinds of metal salts such as lithium salts are used.
Non-metallic compound accelerators are also known, and organic phosphorus compounds or organic nitrogen compounds are typical. Further, combination of organic phosphorus compounds with metal compounds, such as chromium hexacarbonyl, is known (Japanese Patent Laid open No. 15403/1976).
Since, in the preparation of acetic anhydride by carbonylation, methyl acetate and acetic anhydride themselves act as a solvent, no other solvents are particularly added in most cases, as in the prior arts mentioned above. Examples wherein solvents are used, however, are also known. For example, Japanese Patent Laid open No. 47922/1975 discloses hydrocarbons, ethers, ketones, and fatty acids as inert solvents.
Aliphatic carboxylic acids sometimes serve as more than mere inert solvents. For example, Japanese Patent Laid-open No. 28980/1980 discloses cocatalysts consisting of a combination of picolinium salts with acids and, further, Japanese Patent Laid open No. 57733/1981, No. 99437/1981, and No. 99438/1981 disclose examples in which the presence of acids is essential. In all these examples, compounds containing nitrogen, phosphorus, or arsenic are essential besides the carboxylic acids.
Organic compounds such as organic phosphorus compounds or organic nitrogen compounds and metal compounds such as chromium hexacarbonyl, which are more effective among the accelerators used in the prior arts, are lacking in thermal and chemical stability as well as being expensive, and hence special devices are required in the industrial application of these accelerators to maintain the catalytic activity of them (see Japanese Patent Laid-open No. 51036/1980).